Machaerus cut down Neoptolemus with his sacrificial knife for insulting Apollo at Delphi by burning down his temple. (In fact Orestes was planning to kill Neoptolemus but Apollo prevented him, foreseeing that he must die by the hand of someone else.) The name is a purely descriptive one, since it means 'butcher', mageiros (or else, also appropriately, 'large knife', 'sabre', machaira).